The present invention relates to an integrated circuit comprising at least one linear array of laser diodes, the substrate of which is modified in order to improve the current supply of the semiconductor lasers. The invention also relates to a method for modifying the substrate of this integrated circuit of lasers.
The supply of current to semiconductor lasers, whether these are taken separately or in a group on a linear array containing up to about thirty of them, does not entail any particular problems because the current can flow through the doped, hence conductive, substrate. In this case, the electrical supply is applied to both the upper face and the lower face of the laser or of the linear array.
This is not so with integrated circuits of lasers in which the substrate supports a plurality of linear arrays of lasers alternated with a plurality of bands or surfaces of reflectors. An integrated circuit such as this is described in the French patent application No. 88 14799 dated 15th Nov. 1988 which is the priority document for U.S. Pat. No. 5,012,477. In these integrated circuits, the lasers of a same linear array are supplied in parallel, but the arrays are supplied in series, by metallizations that run along the surface of the semiconductor layers. This type of parallel/series supply is preferable because it is easier to regulate an average current under an average voltage than a high current under low voltage: for example 15 A under 66 V rather than 500 A under 2 V. Consequently, the substrate of the integrated circuit is preferably semi-insulating, otherwise it would short-circuit the linear arrays in series.
The current is applied to the linear arrays of lasers by a first metallization, on the upper face, but the circuit is looped by a conductive layer, for example made of n type GaAs, epitaxially grown on the semi-insulator substrate, electrically linked to a second metallization. To let through the necessary current, this conductive layer should be thick. However, it is epitaxially grown and its thickness cannot exceed about 5 microns. Indeed, the structure of the different layers that are epitaxially grown on the substrate to make an integrated circuit of lasers reaches a thickness of 13 microns, which is considerable. Epitaxial growth on a greater thickness would result in lowering the crystallographic quality.
Furthermore, this conductive layer made of n type GaAs, doped at 2.10.sup.18 a/cm.sup.3 and having a thickness of 5 microns, has electrical resistance of the order of 4 ohms: it is desirable to be able to reduce its resistance.
The invention provides a solution to these problems and can be used to obtain a thick conductive layer with low electrical resistance on a semi-insulator substrate, without lowering the crystallographic quality of the structure. This is obtained by a thermal diffusion or an implantation designed to dope the substrate on the surface to make it conductive on a given thickness without causing deterioration in its monocrystal crystallography. After this modification of the substrate, the constituent layers of the lasers are epitaxiated, normally, within limits of thicknesses that do not lower the quality of the crystallography.